Scientists figure out our place in Milky Way

Triangulation yields a distance to our galaxy’s nearest spiral arm

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This diagram shows the position of the sun in relation to the various spiral arms of our Milky Way galaxy, as seen from above the galactic "north pole." Each hatch mark along the side of the diagram represents 5 kiloparsecs, or 16,300 light-years.

Astronomers can provide detailed images of beautifully swirling galaxies millions of miles away. It’s our own galaxy they haven’t been able to get their arms around.

“It’s clear that the Milky Way has a structure like those, but it’s hard to tell being in it,” said Mark Reid of the Harvard-Smithsonian Center for Astrophysics.

Now, a team of astronomers has taken an important step toward mapping the Milky Way by accurately measuring the distance to the star-forming region W3OH in the Perseus spiral arm, the nearest arm to us. This long strand of stars streaks out of the Milky Way’s disk in the same manner as others seen in galaxies across the universe.

Until now scientists had difficulties figuring just how far away spiral arms are, and various measurements and techniques had discrepancies ranging by a factor of two.

The new results are from a telescope nearly the size of Earth. The astronomers used the Very Long Baseline Array, taking observations from several telescopes stretching from Hawaii to the Virgin Islands, to create the resolution of a telescope nearly 5,000 miles (8,000 kilometers) in diameter.

“We have established that the radio telescope we used, the Very Long Baseline Array, can measure distances with unprecedented accuracy — nearly a factor of 100 times better than previously accomplished,” Reid said.

Although the VLBA has extremely high resolution, the source objects need to be very bright and compact. Spiral arms are just that — bright clouds of star-forming gas. In particular, this team focused in on a cloud of methyl alcohol surrounding a newly formed star.

“We measured distance by the simplest and most direct method in astronomy — essentially the technique used by surveyors called triangulation,” said team member Xu Ye.

NASA/JPL-Caltech/R. Hurt (SSC/Caltech)

An artist's rendering of the Milky Way and its central bar structure, with the sun's position noted.

This technique, also known as parallax, comes straight out of a high-school geometry textbook. In this case, the researchers used the changing vantage point from Earth as it orbits the sun to form one leg of the triangle. Then, they calculated the star’s distance by measuring the change in its position as Earth orbits the sun.

The astronomers found that this young star is actually moving around the Milky Way in an orbit that is about 10 percent off from being circular. It appears to rotate slower than other stars in circular orbits and is falling toward the center of the Milky Way.

Although scientists don’t really understand the details of spiral arm formation, these observations support the “spiral density-wave theory.” This theory suggests that a combination of gravitational instabilities and shear forces — a result of the outer edge of the galaxy moving slower than the inner — causes material to cluster and eventually shoot off in an arm.

The researchers are now shifting the VLBA’s gaze toward three other spiral arms, where they will look at about a dozen star-forming areas similar to W3OH.

This research is detailed Thursday in the online version of the journal Science.